Known in the art is a method for preparing 2,4-diaminophenol by treating 2,4-diaminophenol sulphate with an alkali (cf. Kirk-Othmer Encyclopedia of Chemical Technology, v.2, Interscience Publishers, New York, 1963). In turn, 2,4-diaminophenol sulphate is prepared by electrolytically reducing 1,3-dinitrobenzene (cf. German Pat. No. 75,260) and 3-nitroaniline (cf. German Pat. No. 78,829).
2,4-Diaminophenol sulphate is produced from 1,3-dinitrobenzene upon reduction on a copper cathode or a monel-metal cathode in sulphuric acid in the presence of bivalent tin sulphate or tetravalent germanium sulphate at a temperature of from 90.degree. to 95.degree. C. (cf. Day, Udupa, J. Scient, Ind. Res., India, 6B, 83-92, 1947). Instead of bivalent tin sulphate or tetravalent germanium sulphate in the electrolytical reduction use can be made of bivalent iron sulphate (Kirchgof, Chim. farm. Promysl, 1933, 326, 329). In a similar manner it is possible to produce 2,4-diaminophenol sulphate through electrochemical reduction of 2,4-dinitrophenol (Dey, Maller, Pai, J. Scient., Ind. Res., India, 7B, 71, 74, 1948) in sulphuric acid at the temperature of 90.degree. C.
The electrolytical reduction of 2,4-dinitrophenol occurs according to the following scheme: ##STR1##
In prior art methods of electrolytical reduction 2,4-diaminophenol sulphate is obtained in a low yield equal to 48-55% of the theoretical value. Furthermore, separation of the desired product from the resulting by-products is associated with considerable technical difficulties.
Therefore, the absence of an efficient method for the preparation of 2,4-diaminophenol sulphate makes it now inexpedient to commercially use it as the starting material for the production of 2,4-diaminophenol. The yield of 2,4-diaminophenol by the process comprising treating 2,4-diaminophenol sulphate with an alkali is 44 to 50% based on the initial 2,4-dinitrophenol.
For the same reasons, it is economically inexpedient to produce 2,4-diaminophenol dihydrochloride by the process based on the use of the same 2,4-diaminophenol sulphate as the starting material. The process comprises treating 2,4-diaminophenol sulphate with calcium chloride, followed by precipitation of the resulting 2,4-diaminophenol dihydrochloride by means of hydrochloric acid.
2,4-Diaminophenol dihydrochloride can be also produced by treating 2,4--diaminophenol with hydrochloric acid (Kirk-Othmer Encyclopedia of Chemical Technology, v.2, Interscience Publishers, New York, 1963) produced by the above-described method. The recovery and purification of 2,4-diaminophenol dihydrochloride cause additional losses of the desired product and a reduced yield thereof (to 42-48% based on the starting 2,4-dinitrophenol).
Known in the art is a process for producing 2,4-diaminophenol dihydrochloride by reducing 2,4-dinitrophenol in a hydrochloric-acid medium by means of hydrogen evolving upon the effect of hydrochloric acid on iron shavings. In this process the reduction of 2,4-dinitrophenol is conducted at a temperature within the range of from 50.degree. to 70.degree. C. in a medium of a 20-30% hydrochloric acid in the presence of iron shavings (cf. German Pat. No. 269542; J. Schwyzer, "Die Fabrication pharmazeutischer und chemisch-technischer Produkte", Berlin, 1931, s. 195). To prevent oxidation of the resulting 2,4-diaminophenol dihydrochloride there is added an aqueous solution of tin dichloride. On completion of the reaction the resulting commercial product is precipitated by the addition of hydrochloric acid and filtered-off. The yield of the commercial product is 55.8% as calculated for the starting 2,4-dinitrophenol. From 40 g of the commercial product there are isolated, by purification with active coal and precipitation with hydrochloric acid, 22 g of pure 2,4-diaminophenol dihydrochloride.
This prior art method has a disadvantage residing in high losses of the product thus causing a low yield of 2,4-diaminophenol dihydrochloride (27-30% based on the starting 2,4-dinitrophenol). Furthermore, a large amount of waste waters containing harmful products are formed in the process.
It is possible to produce 2,4-diaminophenol) from 2,4-diaminophenol dihydrochloride prepared by the above-described method by treating the dihydrochloride with an alkali.
However, in this case the yield of 2,4-diaminophenol is not more than 30% based on the starting 2,4-dinitrophenol which is below its yield (44-50%) in the earlier described prior art process comprising treatment of 2,4-diaminophenol sulphate with an alkali.